1. Field of the Invention
The present invention relates to an imaging apparatus, such as a video camera or a digital camera, and more particularly to the gradation correction of luminance level signals.
2. Description of the Related Art
An imaging apparatus, such as a video camera or a digital camera, conventionally obtains an image signal using an image sensor, such as a charge-coupled device (CCD) sensor or a complementary metal-oxide semiconductor (CMOS) sensor. Such an image sensor has a narrow range of gradation expression, which expresses from black level to white level. Therefore, the obtained image signals cannot sufficiently express the gradation range of an actual object image. The gradation range is further limited in an expression range of the digital signals in a system, in which image signals are digitalized and recorded. In order to solve the problem, conventionally, knee processing, which compresses the high luminance range of image signals to expand the gradation range to be expressed, is widely used.
Input/output characteristics and the gradation range of the knee processing are described referring to FIG. 5. In FIG. 5, the horizontal axis indicates an input signal value while the vertical axis indicates an output signal value after the knee processing is performed. Respective straight lines indicate input/output characteristics in a case where the knee processing is varied. In a case where no knee processing is executed, the input/output characteristics are expressed in a straight line indicated by a, that is, an output signal value is similar to an input signal value. In other words, the range of the input signal value Ia is equal to the range of the output signal value Oa. In a case where the knee processing is executed, the input/output characteristics transfer from the straight line a to a straight line b or c. In the characteristics of the straight line b, the output signal values are similar to the input signal values up to the input signal value Ib, similar to the case where no knee processing is executed. In contrast, the gradation of the output signal values is compressed in the high luminance range where the input signal value is larger than the value Ib. As a result, the input signal range between values 0 and Ia is compressed in the output range between values 0 and Ob. In the case of the straight line c, the gradation of the output signal values is similarly compressed in a high luminance range where the input signal value is larger than the value Ic. As a result, the input range between values 0 and Ia is compressed in the output range between values 0 and Oc.
Thus, by applying the knee processing to the input signals, the input signals can be compressed, and the gradation range to be expressed practically can be expanded. The starting points kb and kc of the polygonal lines in the respective characteristics are referred to as a knee point and the slope of the straight line after the knee point is referred to as a knee slope. When the knee processing is to be intensified, shifting the knee point to the lower side or reducing the angle of the knee slope is generally executed.
By executing the knee processing as described above, the gradation range for an object in a backlight state or for an image including a high luminance object can be corrected. Setting an amount of correction, that is, the knee intensity is also executed depending on the image state of the object. For example, Japanese Patent Application Laid-Open No. 03-204281 discusses a method for setting the knee intensity based on the detection result of a plurality of light metering areas. Japanese Patent Application Laid-Open No. 03-285470 discusses a method for setting the knee intensity based on an average luminance signal of the entire image plane and a luminance signal in the center of the image plane.
According to the method described in Japanese Patent Application Laid-Open No. 03-204281, however, an effective knee intensity cannot be set in a case where only the luminance of the object in the center is high and the luminance of the surroundings is dark, while the knee intensity can be set effectively in a case where the luminance of the surroundings such as the sky is high. According to Japanese Patent Application Laid-Open No. 03-285470, the knee processing is determined based only on a difference between the average luminance of the entire image plane and the luminance of the center of the image plane. Thus, appropriate knee processing cannot be performed in a case where both of the luminance of the surroundings, such as the sky, and the luminance of the object in the center of the image plane are high.